1. Field of the Invention
The present invention relates to a film formation apparatus for a semiconductor process for forming a film on a target substrate, such as a semiconductor wafer, and also to a method of using the apparatus. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
In manufacturing semiconductor devices, a process, such as CVD (Chemical Vapor Deposition), is performed to form a thin film, such as a silicon nitride film, on a target substrate, such as a semiconductor wafer. For example, a film formation process of this kind is arranged to form a thin film on a semiconductor wafer, as follows.
At first, the interior of the reaction tube (reaction chamber) of a heat-processing apparatus is heated by a heater at a predetermined load temperature, and a wafer boat that holds a plurality of semiconductor wafers is loaded. Then, the interior of the reaction tube is heated up to a predetermined process temperature, and gas inside the reaction tube is exhausted through an exhaust port, so that the pressure inside the reaction tube is reduced to a predetermined pressure.
Then, while the interior of the reaction tube is kept at the predetermined temperature and pressure (kept exhausted), a film formation gas is supplied through a process gas feed line into the reaction tube. For example, in the case of CVD, when a film formation gas is supplied into a reaction tube, the film formation gas causes a thermal reaction and thereby produces reaction products. The reaction products are deposited on the surface of each semiconductor wafer, and thereby form a thin film thereon.
Reaction products generated during the film formation process are deposited (adhered) not only on the surface of the semiconductor wafer, but also on, e.g., the inner surface of the reaction tube and other members, the latter being as by-product films. If the film formation process is continued while by-product films are present on the interior of the reaction tube, some of the by-product films separate therefrom and generate particles. The particles may drop on the semiconductor wafer, which reduces the yield of semiconductor devices being fabricated.
In order to solve this problem, cleaning of the interior of the reaction tube is performed after the film formation process is repeated several times. In this cleaning, the interior of the reaction tube is heated at a predetermined temperature by a heater, and a cleaning gas, such as a mixture gas of fluorine and a halogen-containing acidic gas, is supplied into the reaction tube. The by-product films deposited on the inner surface of the reaction tube are dry-etched and removed by the cleaning gas. Jpn. Pat. Appln. KOKAI Publication No. 3-293726 discloses a cleaning method of this kind. However, as described later, the present inventors have found that conventional cleaning methods of this kind have some difficulty in performing cleaning for an optimum cleaning time period (which varies depending on the conditions inside a reaction tube).